1. Field of the Invention
The present invention relates to novel glass compositions suitable for fiberization by rotary and other discontinuous fiber processes which have a unique and useful set of properties that provide advantages over current technology in both manufacturing and in product end use. The fibers prepared are biosoluble, while also exhibiting a large working range for fiberization.
2. Description of the Related Art
Glass forming compositions suitable for fiberization are typically restricted by their melt and end properties to conform to process specifications and product performance criteria. For example, in production processes used for making discontinuous glass fibers, only certain values for fiberization viscosity and liquidus are acceptable. Moreover, such compositions must demonstrate adequate physical properties such as tensile strength and moisture resistance when formed into fibers by these processes. In addition, recently, it has become increasingly more important that these fibers degrade at sufficiently high rates in the body, such that they pose little to no potential risk to humans if inhaled and can at least be demonstrated to have limited biological effects to laboratory animals when tested.
A good discontinuous glass fiber forming composition should also have good “runability”—the ability to be easily fiberized into long fibers of small diameter with good production rates and little or no shot. While there are many factors involved in this, not all of which have been clearly identified, it is believed that surface tension and lack of tendency for the melt to phase separate or crystallize play key roles. In specific, it is desirable for a glass composition to have as low a surface tension as possible at fiberization temperatures (keeping in mind the other factors above), such that the work done in forming a unit area of surface is kept to a minimum.
The glass must be capable of being melted and fiberized at temperatures low enough for the capability of the equipment and to attain reasonable (economic) production rates. For discontinuous fibers produced by current rotary technology this requires that the HTV value of the glass (temperature when the melt viscosity is 100 Pa s) be less than 1135° C. (2075° F.), preferably less than 1090° C. (1994° F.), and the surface tension of the glass at 1000° C. (1832° F.) be less than 300 dyne/cm.
The glass must not crystallize or devitrify in the melters, forehearths, spinners or anywhere in the system used to melt, contain, transport, or fiberize the glass. Crystallization impairs flow of melt to fiberization orifices. To prevent devitrification and provide a good working range for discontinuous fiberization, the liquidus temperature of the glass must be at least 55° C. (100° F.) below the HTV temperature. However, preferably this difference or working range should be as large as possible to insure smooth, efficient operation and produce long fibers with fine diameters and the least amount of shot. Glasses with working ranges of >200° C. and most preferably >235° C. are most desirable.
In addition to crystallization or liquidus temperature, crystallization rate is also a highly desirable attribute of a discontinuous fiber-forming composition. Here, as low a rate as possible is most desirous. For example, low devitrification rates allow operation under both high temperature conditions where efficient, high throughput production as described above is required or under low temperature conditions when high throughput or volume is not needed or when energy savings become more important. Thus a glass which is capable of both is more flexible in its application and has distinct advantages over more conventional formulations. For best performance, the glass should show crystals no larger than 5 μm in size after 24 hours exposure in a liquidus boat per ASTM C829-81 methodology and crystals no larger than 25 μm in size after 72 hours exposure.
As some applications for discontinuous fiber require the fiber to remain in tact during continual exposure to high temperatures without softening or drooping, the glass must have a reasonably high softening point. To best meet criteria for the majority of these applications, the softening point should be at least 675° C. (1247° F.).
A major application of discontinuous fibers is in thermal insulation. Glasses useful in these applications must be capable of being drawn into fine diameters (less than 1 μm) without breakage into very short lengths (<1 mm) that would render them inferior in regard to providing structural support to the product. As noted above, glasses with large working ranges facilitate this capability.
Because of its high specific surface area, glass fiber must be sufficiently chemically durable, particularly with regard to ambient atmospheric moisture, so that little to no deterioration in fiber strength occurs with time during packaging, shipping, and storage prior to end use. The moisture resistance of a glass fiber can be correlated with its thermodynamic tendency to react with water as measured by its free energy of hydration (ΔGhyd). For nearly all commercial glass fiber, this value is negative, indicating that the fibers will over time react. Large negative values of ΔGhyd represent a strong tendency to degrade by reaction with moisture which frequently correlates with high rates of hydration reactions. This is critical for discontinuous fibers used in building, mechanical, automotive, aircraft or OEM equipment applications which provide insulation and structural performance. For fiber to perform adequately and preserve tensile properties in moist or humid environments, the free energy of hydration should not be less than (more negative than) −6.0 kcal/mole as calculated by the methods described in U.S. Pat. No. 5,401,693, which is herein incorporated by reference in its entirety.
Because the fiber is of very fine diameter and capable of being respired, it must also degrade in the body at a rate that is sufficiently rapid so as not to induce respiratory diseases, especially chronic diseases such as emphysema or cancer. Measured in vitro dissolution rates for the fiber in simulated physiological saline (kdis) must be greater than 100 ng/cm2 hr. and more preferably greater than 250 ng/cm2 hr. More importantly, the fiber must demonstrate capability to decompose in vivo. An accepted evaluation is that provided by EU protocols ECB/TM 26 or TM 27 rev. 7. For ECB/TM 27 rev. 7. (exposure by intratracheal instillation), fibers of greater than 20 μm in length must be cleared from the lung at a rate sufficient to achieve a fiber biopersistence half life (t ½) of less than 40 days. Glass fiber compositions that are particularly desirable are those that can far exceed fiber clearance rates in animal tests specified by these methods, i.e. those with a good “safety margin”.
There is a need in the industry for glass compositions and products made therefrom which exhibit the unique combination of properties as discussed above. While many current glass fiber formulations provide one or more of the requisite properties noted, the industry is still searching for the glass compositions which provide the glass chemistries necessary to meet all of the foregoing manufacturing and product performance needs. It is an object of the present invention to provide such glass compositions and products made therefrom.